This view is confirmed by the fact that we only feel images to be
"unreal" when we already know them to be images. Images cannot be
defined by the FEELING of unreality, because when we falsely believe an
image to be a sensation, as in the case of dreams, it FEELS just as real
as if it were a sensation. Our feeling of unreality results from our
having already realized that we are dealing with an image, and cannot
therefore be the definition of what we mean by an image. As soon as an
image begins to deceive us as to its status, it also deceives us as to
its correlations, which are what we mean by its "reality."

(3) This brings us to the third mode of distinguishing images from
sensations, namely, by their causes and effects. I believe this to be
the only valid ground of distinction. James, in the passage about the
mental fire which won't burn real sticks, distinguishes images by their
effects, but I think the more reliable distinction is by their causes.
Professor Stout (loc. cit., p. 127) says: "One characteristic mark of
what we agree in calling sensation is its mode of production. It is
caused by what we call a STIMULUS. A stimulus is always some condition
external to the nervous system itself and operating upon it." I think
that this is the correct view, and that the distinction between images
and sensations can only be made by taking account of their causation.
Sensations come through sense-organs, while images do not. We cannot
have visual sensations in the dark, or with our eyes shut, but we can
very well have visual images under these circumstances. Accordingly
images have been defined as "centrally excited sensations," i.e.
sensations which have their physiological cause in the brain only, not
also in the sense-organs and the nerves that run from the sense-organs
to the brain. I think the phrase "centrally excited sensations" assumes
more than is necessary, since it takes it for granted that an image must
have a proximate physiological cause. This is probably true, but it is
an hypothesis, and for our purposes an unnecessary one. It would seem to
fit better with what we can immediately observe if we were to say that
an image is occasioned, through association, by a sensation or another
image, in other words that it has a mnemic cause--which does not prevent
it from also having a physical cause. And I think it will be found that
the causation of an image always proceeds according to mnemic laws, i.e.
that it i